A heavy-duty engine was modified to operate on natural gas using a partially-premixed charge strategy. The 15 L engine used a production natural gas fuelling system which was capable of providing direct injections of natural gas and diesel at high pressure during the intake stroke and around TDC of the compression stroke. The engine's compression ratio was reduced to 15.3:1 to maximize load without exceeding the peak cylinder pressure or encountering knock. A multi-mode strategy for the natural gas injection was used: at part-load the injection occurred during the intake stroke, generating a premixed charge, while at high load a second injection was added around TDC to generate a non-premixed combustion phase. Using this strategy, loads up to 19 bar BMEP were achieved with brake efficiencies of nearly 40% and NOx emissions below 0.29 g/kWh. The key parameters needed to achieve the target load without knock were EGR level, premixed EQR, and intake manifold temperature. At high load, smoke emissions were significant, while at part load, high efficiency and low NOx were achieved but unburned fuel emissions increased. CFD simulation results indicated that the part-load barriers were a result of slow flame propagation through the lean premixed mixture. The modelling suggested that methods to overcome this could include partial-premixing and increased turbulence during the later stages of the combustion.
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